The proper fluids for drilling, gravel packing and sand screens installation are essential for well completion success. Careful planning, well preparation and completion execution are required to increase completion productivity and longevity. Historically, a minimum of three fluids has been used to drill and complete gravel packed wells. The first fluid is a solids-laden drilling-fluid used to drill the completion interval. The second fluid is a solids-free completion-fluid used to displace the solids-laden drilling-fluid and to run sand-exclusion equipment and gravel packing tools in a generally solids-free environment. The third fluid is a carrier fluid for the gravel during gravel packing of the completion interval.
In producing hydrocarbons a wellbore is drilled through a subterranean reservoir. Drilling practices can affect a gravel pack and sand screen the same way they can affect conventionally perforated wells. The well should be drilled to maintain wellbore stability, and drilling fluids should be used that will not damage the formation.
The drilling fluid typically contains weighting solids, viscosifying solids, and drilled solids at varying concentrations. Drilling fluid filtrates should be compatible with completion fluids and should not interfere with the completion operations. Preferably, the drilling fluid selected should be dense enough to result in a well that is slightly overbalanced, should have low fluid loss and should be compatible with the clays in the productive formation.
The proper preparation of a well for gravel packing can be the key to completion success. Cleanliness is one of the most important considerations in the preparation of gravel packs. The presence of any particulate materials can result in a damaged completion. Currently tanks are often dedicated to gravel pack use to avoid repeated cleaning operations for drilling mud removal.
Completion fluids are used to displace the solids-laden drilling fluid and to run sand-exclusion equipment and gravel packing tools in a generally solids-free environment. Completion fluids can be oil- or water-based fluids. The water-based fluids are usually considered to be more flexible. Their densities, viscosities, and formation compatabilities are more easily controlled than those of oil-based fluids. Therefore, water-based fluids are most commonly utilized.
Regardless of the source of the completion fluid, the fluid should contain minimum particulate material and its chemistry must be compatible with the rock formation and connate water. Fresh water may cause clays to swell or disperse, while the presence of some ions may cause precipitation when in contact with formation water. The most common sources of completion fluids are field or produced brine, seawater, bay water or fresh water. The density of the completion fluids is often controlled with soluble salts.
Gravel placement involves those operations required to transport gravel from the surface to the completion interval to form a downhole filter that will permit the flow of fluids into the well but will prevent the entry of formation sands. Preferably, the gravel placement provides a uniform pack with a porosity of thirty-nine percent or less.
The gravel placement requires fluid to transport the gravel slurry to the completion interval. Oil- and water-based fluids and foams are commonly used as the gravel placement fluid. Clean fluids are essential for gravel placement. Depending on well pressures, high-density, solids-free soluble salt solutions may be required to maintain well control. In addition, the gravel placement fluids can be viscified by adding polymers.
Poor distribution of the gravel slurry is often caused when carrier fluid from the slurry is lost prematurely into the more permeable portions of the formation and/or into the screen, itself, thereby causing “sand bridge(s)” to form in the well annulus around the screen. These sand bridges effectively block further flow of the gravel slurry through the well annulus thereby preventing delivery of gravel to all levels within the completion interval.
To alleviate poor gravel distribution, “alternate-path” well tools or technology have been proposed and are now in use which provide for uniform distribution of gravel throughout the entire completion interval notwithstanding sand bridges formation before completion of gravel distribution. Such devices typically include perforated shunts or by-pass conduits which extend along the length of the device and which are adapted to receive the gravel slurry as it enters the well annulus around the device. If a sand bridge forms before the operation is complete, the gravel slurry can still be delivered through the perforated shunt tubes (such as, “alternate-paths”) to the different levels within the annulus, both above and/or below the bridge. U.S. Pat. Nos. 4,945,994 and 6,220,345 provides descriptions of typical alternate-path well screens and how they operate.
To summarize, the current method used to install open-hole gravel packs typically involves drilling the completion interval with water- or oil-based drilling fluid, displacing the fluid in the open-hole to a solids-free completion fluid (typically brine), running the gravel pack assembly and sand screens to depth in the solids-free completion fluid, and gravel packing the interval with a water-based carrier fluid. A common limitation of this method involves the inability to run the gravel pack assembly and sand screens to depth due to wellbore instability (collapse) caused by incompatibility between the water-based completion fluid (brine) and the formation. This method is inefficient since at least three fluids are required (drilling fluid, completion fluid, and gravel carrier fluid).
A frequent modification to the method described above involves placing a pre-drilled liner in the completion interval prior to displacing the open-hole to completion fluid and running the gravel pack assembly and sand screens (Murray, G., Morton, K., Blattel, S., Davidson, E., MacMillan, N., Roberts, J., SPE 73727, Feb. 20-21, 2002. Development of the Alba Field—Evolution of Completion Practices, Part 2 Open Hole Completions; Successful Outcome—Drilling with SBM and Gravel Packing with Water Based Carrier Fluid and Gilchrist, J. M., Sutton, Jr., L. W., Elliot, F. J., SPE 48976, Sep. 27-30, 1988. Advancing Horizontal Well Sand Control Technology: An OHGP Using Synthetic OBM.). The pre-drilled liner mitigates wellbore collapse and provides a conduit for running the gravel pack assembly and sand screens. While the pre-drilled liner improves the ability to run the gravel pack assembly and sand screens to depth, it provides an additional resistance to flow and may have a negative impact on productivity.
The current practice of using separate fluids for drilling, displacing the solids-laden drilling fluid and running sand-exclusion equipment and gravel packing tool, and gravel placement is both costly and time-consuming. Accordingly, there is a need to reduce operational complexity and time by simplifying the fluid system and eliminating the need for the pre-drilled liner. This invention satisfies that need.